In electric power equipment such as a gas-insulated switchgear, a high-voltage conductor (electric wire) is supported inside a metal container in which an encapsulated insulating gas is sealed and an insulation state is thereby maintained. An electric field distribution inside the metal container is designed to provide a uniform electric field, but when a defect that forms a non-uniform electric field (e.g., a metal foreign object or a needle-like protrusion) is mixed, there are cases where partial discharge occurs with a portion of the defect as a starting point. When the partial discharge is left untreated, an electrical breakdown may occur to cause an accident, and hence it is important to detect an initial stage of the partial discharge and prevent the electrical breakdown beforehand.
Accordingly, in order to detect the partial discharge, a partial discharge sensor is provided. It is known that when the partial discharge occurs, an electromagnetic wave of a VHF band to a UHF band is emitted. As a conventional partial discharge sensor, a partial discharge sensor that detects the discharge by receiving the electromagnetic wave is proposed. For example, Patent Document 1 discloses an antenna device for insulation monitoring in which a slot antenna is constituted by forming a closed loop circuit in a high-frequency manner with a pair of flanges at a junction portion of metal containers and a plurality of studs for coupling the pair of flanges, whereby an electromagnetic wave leaked from a gap of the junction portion of the metal containers to the outside is received.